A Computational Assessment of Laminar Flame Speed Correlation in an Ultralean Prechamber Engine

Abstract

Predictive modeling of pre-chamber combustion engines relies primarily on the correct prediction of laminar and turbulent flame speeds. While the latter has been rigorously derived, the former correlations are mostly semi-empirical and valid for a limited range of operating conditions. The current work aims at highlighting the fundamental significance of correct laminar flame speed prediction on numerical modeling of ultralean prechamber engine combustion. Gulder's empirical correlation for laminar flame speed was chosen for the current work. It was modified for ranges beyond what it was originally derived for. It was initially observed that the numerical results that utilize Gulder's correlation for the laminar flame speed underperform compared to the one computed from the skeletal GRI3.0 by Lu and Law. In all cases, Peters' turbulent flame speed correlation was used, which evidences that any potential difference comes from the laminar flame speed. Using Lu and Law's chemical mechanism as a reference for laminar flame speed calculations, the values of the empirical constants α, η, and ξ in Gulder's correlation were optimized to yield more accurate flame speeds at ultralean engine conditions. The updated Gulder's correlation for methane was implemented in CONVERGE, a three-dimensional computational fluid dynamics (CFD) solver, and validated against the experimental engine results from KAUST. The flame topology was also explored to correlate the observed behaviors in the pressure predictions among all tested cases. Finally, the Borghi-Peters diagram provides insightful information on combustion regimes encountered in pre-chamber combustion engines.